This is a good question, which can be answered in a few different ways. Simon is unique in the book. He is the one character who is consistently good. He is the "religious" one among the boys.There is a sense of innate goodness to him. In fact, he is the only one that is not influenced by social conditioning to do the right thing. He does good things, simply as part of his nature. The fact that he is the only one like this suggests that this type of morality is very rare indeed. From a symbolic level, in chapter 9 he dies as a sacrifice.
When it comes to Piggy, he is the intellectual, who stands by Ralph the leader. He is innovative and filled with great ideas, but the reader does not get the sense that he is like Simon in his nature. If anything he is the more rationalistic side of humanity, which can be cold and utilitarian. Moreover, he is completely ineffectual without Ralph. For these reasons, he is different than Simon. The chief similarity is that they both die.